CD8+ T cells play a central role in immunity against viruses (and other intracellular organisms), and their efficacy is determined by the cumulative effect of at least three factors, which are studied in this proposal: effector function, abundance and tissue distribution. There are 4 specific aims: (1) How are CD8+ T cell effector functions regulated in various tissues (lymphoid and non-lymphoid), over the course of virus infection, and after re-exposure to antigen in vivo? We have developed a novel technique - the in vivo intracellular cytokine stain - that allows us to identify CD8+ T cells that are actively producing cytokines in response to in vivo antigen contact. Over the course of virus infection, we will assess the constitutive expression of CD8+ T cell effector molecules (IFNgamma, TNF & perforin) in vivo, in various tissues (lymphoid & non-lymphoid); and we will determine how rapidly (minutes / hours) the effector molecules are induced in the various tissues following in vivo contact with antigen. (2) How quickly do virus-specific CD8+ memory T cells exert their antiviral effects in the various tissues? We anticipate that effector molecules will be expressed within minutes of antigen contact; are their antiviral effects exerted over the same very short time-frame, in virus-infected mice? (3) How is immunodominance regulated by IFNgamma? Is there a selective advantage favoring cells which produce this cytokine most rapidly? Does this cytokine deliver an autocrine stimulatory signal, causing the fastest-producing cells to predominate? Over what physical range can a CD8+ T cell exert IFNgamma-mediated effects on other cells (is the effect paracrine or endocrine)? (4) How is the abundance of memory T cells regulated in the various tissues? Do CD8+ T cells proliferate in response to antigen contact only in lymphoid tissues, or do they also divide when they encounter antigen elsewhere? Can we increase the number of memory cells, thus enhancing protective immunity?